Filter bags comprising a porous membrane

ABSTRACT

A filter bag is disclosed that comprises a porous membrane having a strength in the transverse direction to improve durability. There is a filter assembly for filtering particulates from a gas stream comprising a support substructure and a filter bag at least partially surrounding the support substructure. The filter bag comprises a porous membrane having a upstream surface exposed to the gas stream. The porous membrane is lightweight and has a structure to collect the particulates on the upstream surface. In particular, the porous membrane has a bubble point of 0.06 MPa or more and has a strength in a transverse direction that is 100 N/m or more. Other filter bags disclosed comprise a laminate comprising a porous membrane having a bubble point of 0.06 MPa or more and a second layer that acts as a sacrificial material.

TECHNICAL FIELD

The present invention relates generally to a filter bag comprising aporous membrane. In particular, the porous membrane may be attached to afilter media or laminated with a second layer that is attached to thefilter media. The porous membranes are durable to resist stress incurredby cleaning cycles.

BACKGROUND

The removal of particulates from a gas stream has long been a practicein a variety of industrial fields. Conventional means for filteringparticulates and the like from gas streams include, but are not limitedto, filter bags, filter tubes, filter cartridges and filter panels.These filter elements are typically oriented into a filtration system,often referred to as a filter baghouse, for filtering such particulates.Such filtration systems may be either cleanable or non-cleanable,depending on the requirements of the system operation.

The separation of particulate matter from industrial fluid streams isoften accomplished using laminate filters. These textile-based laminatefilters remove particulates from fluid streams. When the resistance toflow or pressure drop through the textile caused by accumulation ofparticulate on the filter becomes significant, the filter must becleaned, and the particulate removed from the filter.

It is common in the industrial filtration market to characterize thetype of filter bag by the method of cleaning. The most common types ofcleaning techniques are reverse air, shaker, and pulse-jet. Reverse airand shaker techniques are considered low energy cleaning techniques.

In reverse air filtration techniques, particulate collects on theinterior of the bag. During cleanings, a gentle backwash of aircollapses the bag and fractures the dust cake off the bag, which exitsthe bottom of the bag into a hopper.

Shaker mechanisms also clean dust cakes that collect on the inside of abag. The top of the bag is attached to an oscillating arm which createsa sinusoidal wave in the bag to dislodge the dust cake.

In pulse-jet filtration, the particulate is captured on the outside ofthe bag. Pulse-jet cleaning techniques employ a short pulse ofcompressed air that enters the interior top portion of a filter bag ortube. The energy of this cleaning pulse expands the bag, knocking offthe dust cake. The bag will typically snap back to a cage support and goright back into service collecting particulate.

Of the three cleaning techniques, pulse-jet is the most stressful on thefilter media. However, in recent years, industrial process engineershave increasingly selected pulse-jet baghouses for dust collectionapplications because of:

-   -   1. Smaller unit size (sometimes as much as ½ or ¼ the size of        shakers and reverse air filtration) due to:    -   (A) higher volumetric airflow/cloth area ratio (higher operating        velocity through media); and    -   (B) on-line cleaning allows the unit to be designed at the        desired flow rate, hence there is no need for additional filter        media area to allow for off-line cleaning.    -   2. Minimal number of moving parts.    -   3. Lower number of bags to replace.

In a pulse-jet baghouse, bags are inserted into the baghouse with ametal cage on the inside to keep them from collapsing. Dirty aircontaining dust enters the baghouse on the outer side of the bag wherethe dust accumulates on the surface. The cleaned air travels through thebag and out of the baghouse. When a sufficient amount of dust hasaccumulated on the outside of the bag to cause a decreased amount of airflow through the bag, the pulse-jet baghouse sends a pulse of highpressure air backwards through the bag. The accumulated dust is forcedoff the bag for collection in the lower portion of the baghouse by acombination of the high pressure air and the movement in the bag causedby the back pulse. This cleaning process may occur multiple times anhour to maintain sufficient air flow through the bag.

The movement in the bag mentioned above is a result of high pressure airimparting a stress on the bag.

U.S. Pat. No. 6,110,243 is directed to filter bag assemblies comprisinga support structure, such as a support cage of metal, plastic, or thelike, and a filter media of expanded PTFE membrane(s) without a backingmaterial or layer. As described, the filter bag assembly furthercomprises a support cover, or cage cover, which fits over the exteriorsurface of the support, or cage, to prevent contact of the filter mediawith the cage.

A continuing problem of improving durability exists for filter bagassemblies. Thus, it is apparent that it would be advantageous toprovide an improved filter bag assembly directed to overcoming one ormore of the limitations set forth above.

BRIEF SUMMARY OF SOME EXAMPLE EMBODIMENTS

According to one embodiment of the present invention, there is provideda filter assembly for filtering particulates from a gas streamcomprising: a filter bag comprising a filter media and a porous membranehaving an upstream surface exposed to the gas stream and a downstreamsurface adjacent to the filter media, wherein the porous membrane has abubble point of 0.06 MPa or more, e.g. 0.09 MPa or more, and wherein theporous membrane has a strength in a transverse direction that is 100 N/mor more, e.g., 175 N/m or more, wherein the filter assembly iscleanable. In one embodiment, the porous membrane is capable ofwithstanding structural failures caused by stresses of a cleaningprocess, e.g., shaking or pulsing, to release the collected particulatesfrom the upstream surface of the porous membrane. The porous membranehas a structure to collect the particulates on the upstream surface.Further, the porous membrane may collect more than 97% of theparticulates having a diameter of greater than 0.07 microns from the gasstream.

According to another embodiment of the present invention, there isprovided a filter assembly for filtering particulates from a gas streamcomprising a filter bag comprising a filter media, a porous membranehaving an upstream surface exposed to the gas stream, and second layer,wherein the second layer is disposed between the filter media and porousmembrane, wherein the porous membrane has a bubble point of 0.06 MPa ormore, e.g. 0.09 MPa or more, wherein the filter assembly is cleanable.The porous membrane adjacent to the second layer has a strength in atransverse direction that may be 50 N/m or more, e.g., 100 N/m or moreor 175 N/m or more. In one embodiment, the second layer comprises awoven textile, a non-woven textile, or a membrane. The second layer actsa sacrificial layer and prevents damage to the porous membrane from thefilter media or stresses of the cleaning process. In one embodiment, thesecond layer has a permeability of 10 cfm/ft² @ 0.5 inch gauge of water.The second layer may be laminated to the porous membrane to form alaminate. Alternatively, the second layer may be laminated to the filtermedia.

In one embodiment, the porous membrane comprises a fluoropolymermembrane or a polyester membrane. Along with the high bubble pointand/or high transverse strength, the porous membrane may have one ormore of the following characteristics. The porous membrane may have amass per area ratio of 4.5 gsm or less. The porous membrane may alsohave a Ball Burst strength of 1.36 kg or less. In addition, the porousmembrane may have a permeability of 1 cfm/ft² @ 0.5 inch gauge of water.Further, the porous membrane has a thickness from 5 to 50 microns.

In one embodiment, the filter media comprises a woven felt, non-wovenfelt or a fiberglass material. Depending on the type of bag there may bea support substructure that is adjacent to the filter media.

In another embodiment, there is provided a baghouse filter systemcomprising a housing having an inlet and an outlet, a tube sheetpositioned within the housing between the inlet and outlet, and one ormore of the filter bag assemblies mounted to the tube sheet, wherein thefilter bag assemblies each comprise a filter bag comprising a filtermedia and a porous membrane having an upstream surface exposed to thegas stream and a downstream surface adjacent to the filter media,wherein the porous membrane has a bubble point of 0.06 MPa or more, e.g.0.09 MPa or more, and wherein the porous membrane has a strength in atransverse direction that is 100 N/m or more, e.g., 175 N/m or more.

In further another embodiment, there is provided a baghouse filtersystem comprising a housing having an inlet and an outlet, a tube sheetpositioned within the housing between the inlet and outlet, and one ormore of the filter bag assemblies mounted to the tube sheet, wherein thefilter bag assemblies each comprise a filter bag comprising a filtermedia, a porous membrane having an upstream surface exposed to the gasstream, and second layer, wherein the second layer is disposed betweenthe filter media and porous membrane, wherein the porous membrane has abubble point of 0.06 MPa or more, e.g. 0.09 MPa or more.

These and other embodiments, along with many of their advantages andfeatures, are described in more detail in conjunction with the belowdescription and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood in view of the appendednon-limiting figures.

FIG. 1 is a SEM image of a membrane with structural failures due to alow membrane strength in the transverse direction.

FIG. 2 shows a schematic view of a baghouse in accordance with theembodiments disclosed herein.

FIG. 3 shows a cut-away perspective view of a filter bag with a porousmembrane adjacent to a filter media in accordance with the embodimentsdisclosed herein.

FIG. 4 shows a perspective view of a filter assembly having a filter bagwith a porous membrane adjacent to a filter media in accordance with theembodiments disclosed herein.

FIG. 5 shows a perspective view of another filter assembly having afilter bag with a laminate adjacent to a filter media in accordance withthe embodiments disclosed herein

FIG. 6 is a graph of the rise of differential pressure of a porousmembrane adjacent to a filter media compared with an comparativeexample.

FIG. 7 is a graph of the rise of differential pressure of a porousmembrane adjacent to a filter media compared with an comparativeexample.

DETAILED DESCRIPTION

Persons skilled in the art will readily appreciate that various aspectsof the present disclosure can be realized by any number of methods andapparatus configured to perform the intended functions. It should alsobe noted that the accompanying figures referred to herein are notnecessarily drawn to scale, but may be exaggerated to illustrate variousaspects of the present disclosure, and in that regard, the drawingfigures should not be construed as limiting.

Various embodiments described herein relate to a filter bag comprising afilter media and an adjacent durable porous membrane having a tightmicrostructure that provides improved cleanability. For purposes of thepresent invention, a bubble point of 0.06 MPa or more exhibits a tightmicrostructure. The problem with tight microstructure is that thesemembranes were expected to have significant increases in pressure dropwhich is detrimental to the filter bag. Further thin and/or lightweightmembranes are also expected to have significant increases in pressuredrop which is detrimental to the filter bag. In one embodiment, theporous membrane comprises a thin, tight membrane having a high strengthin a transverse direction. In another embodiment, the porous membrane islaminated to a second layer, such as a membrane. The embodimentsovercome the problems associated with pressure drop issues by using athin and/or lightweight membrane with high strength in the transversedirection or by laminate the thin and/or lightweight membrane to asecond layer leading to improved durability and cleanability of a filterbag. The filter bags described herein may be used in a filter assemblyfor filtering particulates from a gas stream, such as an exhaust gasstream. In one embodiment, a high-efficiency dust-collecting filter bagmay be provided.

During operation the filter bag collects particulates on the upstreamsurface and the collected particulates are removed several times overthe course of operation by cleaning. The filter assembly may be cleanedby removing the particulates that collect as a dust cake on the filterbag using a cleaning techniques such as reverse air, shaker, andpulse-jet. In one embodiment, acoustic cleaning via a sonic horn may beused in combination with these cleaning techniques. Sonic horns used incleaning operations produce a low frequency, high-decibel acousticenergy—sound in the range of 60 to 250 Hz, at intensities up to 150 dB.

Embodiments of the present invention provide an advantage of achievingpermeability while providing increased durability. As membranes becomelighter and/or thinner, the membrane is more vulnerable to damage duringthe manufacturing process and cleaning process. Although structuraldamage in the form of cracks, tears, or ruptures, during themanufacturing process is inconvenient and costly, the membrane can bereplaced prior to use. Once in use, any structural damage on themembrane reduces the lifetime of the filter bag which creates asignificant durability problem.

In addition, further embodiments of the present invention provide asecond layer that acts as a sacrificial layer for the porous membrane.This allows the thin, tight membrane described herein to be used infilter bags with filter media having an abrasive topology or withcleaning techniques that place stress on the filter bags.

The filter bags described herein may be used in a wide variety ofapplications where pollutant control or product capture is necessary.This includes power plants, steel mills, chemical producers, and otherindustrial companies where high particulate collection efficiency isdesired. An exemplary embodiment according to the invention is a filterbag that is used in cement baghouses. Cement baghouses often employfilter media that have abrasive topologies.

An abrasive topology refers to the surface of the filter media havingedges, points, or contours that creates a non-smooth surface. Thisnon-smooth surface is capable of abrading, tearing, rupturing,lacerating, or otherwise damaging an adjacent membrane. Once themembrane is damaged, the filter bag loses efficiency in particulateremoval.

The membrane may also be damaged due to the stresses caused duringcleaning to remove the dust cake. The pulsing during cleaning may createadditional stresses, including abrasive stress, even when the filtermedia has smooth surfaces. The repeated expansion and contraction of thefilter bag against the support substructure and/or the filter mediaduring cleaning may cause damage to the membrane. To overcome theproblems caused by damage, one embodiment of the present invention usesmembranes having a transverse strength that is 100 N/m or more, e.g.,120 N/m or more, 150 N/m or more, 175 N/m or more, or 200 N/m.Transverse strength can be determined using ASTM method D412-06a (2006),using die F. Transverse strength refers to the strength in thetransverse direction on the filter bag, e.g., the direction around thecircumference of the filter bag. As shown in the figures, the transversedirection is indicated by arrow (t). The transverse direction may be inthe direction of the circumferential width of the filter bag which isnon-parallel to the filter bag length, and in some embodiments may beperpendicular to the filter bag length. In one embodiment, thistransverse strength property assists in providing a filter bag that hasan externally facing membrane that is resilient and durable. Membraneswith a transverse strength of less than 100 N/m that adjacent to thefilter media suffer structural failures when subjected to stresses onthe filter bag. As shown in FIG. 1, a membrane 1 having a strength of94.5 N/m in the transverse direction t suffered visible ruptures 2 dueto the lack of transverse strength in the membrane of FIG. 1. Thisindicates that the membrane 1 was less durable and the rupturesshortened the lifetime of the filter bag.

Filter Bag

The filter bags described herein include a membrane that provides highair flow and good dust cake release. This allows the filter bag to beeasily cleaned while at the same time reducing the pressure dropassociated with the filter bag. The membrane may be attached, eitherlaminated or adhered, to a filter media, or the membrane may be adjacentto a second layer to form a laminate that is attached the filter media.The filter bag may have a variety of shapes and features depending onthe operation and cleaning technique. In one embodiment, the filter bagfor pulse-jet cleaning may have a cuff at one end and an opening at theother that allows the filter bag to at least partially surround asupport substructure. Regardless of the design the filter bag having aporous membrane with a high transverse strength helps to increasedurability of the filter bag, which would reduce particulate emissionsto comply with emissions standards.

In a typical baghouse, each filter bag may have a length from 0.3 m to10 m and an internal diameter from 5 cm to 30 cm. The filter bag mayhave dimensions that allow the filter bags to be mounted to thebaghouse, fitted over the support substructure, or fitted with supportsubstructures. It should be understood that these dimensions are notnecessarily limiting for the purposes of the present invention and theembodiments disclosed herein may be used in a wide variety of baghouses.

In one embodiment, the filter bag may be pleated. A pleated filter bagmay increase the depth of the filter media and exposes the filter mediato less particulates in the stream.

Referring now to FIG. 2, a baghouse having a pulse-jet cleaning sequenceis shown. Inside hopper 20, particulate laden gas stream 21 enters thehopper at inlet 22 and passes through filter bag 23 comprising a porousmembrane having a high strength in the transverse direction (t). Thefilter bag may comprise a porous membrane attached to a filter media ora laminate comprising the porous membrane that is attached to the filtermedia. Tube sheet 25 inside hopper 20 prevents the gas stream frombypassing the filter bag 23. The filter bag 23 is kept open by aninternal supporting cage 26 (which is shown under the filter bag 23).The gas stream, after passing through the filter bag 23 and out bag exit29, exits the clean air compartment at outlet 27. In operation,particulate forms a dust cake 28 on the outside of the porous membrane,as shown in the bag on the left of the figure. Upon cleaning to removethe dust cake 28, air from pulse pipe 30 enters the filter bag. Thispulse of air 32 expands the filter bag, loosening the dust cake and thuscausing particulate 31 to collect at the bottom of hopper 20. As seen inthe bag on the right of FIG. 2, the pulse jet causes the filter bag toexpand. The repeated expansion and contraction of the filter bag causeswear. It should be understood that other clean techniques also impartstresses on the filter bags. The advantage of the filter bags describedherein is that the porous membrane has a tight microstructure as well asa higher strength in the transverse direction that improves durabilityand reduces the structural failures caused by stresses during thecleaning cycle.

Porous Membrane

In one embodiment, there is provided a cleanable filter bag assembly forfiltering particulates from a gas stream comprising a filter bag. Thefilter bag comprises a porous membrane having an upstream surfaceexposed to the gas stream and a filter media attached to the porousmembrane. In one embodiment, the porous membrane has a bubble point of0.06 MPa or more, e.g., 0.1 MPa or more or 0.5 MPa or more. In oneembodiment, the porous membrane has a strength in a transverse directionthat is 100 N/m or more, e.g., 150 N/m or more, 175 N/m or more, or 200N/m or more.

The porous membranes used in the filter bags described herein haveseveral characteristics that are particularly advantageous for achievingthe higher performance described herein. The membranes are capable ofcapturing fine particulate, have improved filtration efficiency,increased cake release, and increased air flow capacity in use. In oneembodiment, the porous membrane collects more than 97% of theparticulates in the gas stream having a diameter of greater than 0.07microns. In a further embodiment, the porous membrane collects more than99% of the particulates in the gas stream having a diameter of greaterthan 0.07 microns. When the porous membrane is attached to a filtermedia, the filter bag has a higher collection efficiency.

The porous membranes may comprise a fluoropolymer or a polyestermaterial. A suitable fluoropolymer membrane includespolytetrafluoroethylene (PTFE) that is prepared by a number of differentmethods, including expanding the PTFE to form expandedpolytetrafluoroethylene (ePTFE). Other suitable fluoropolymers mayinclude polyvinylidene fluoride (“PVDF”),tetrafluoroethylene-hexafluoropropylene copolymer (“FEP”),tetrafluoroethylene-(perfluoroalkyl) vinyl ether copolymer (“PFA”), orthe like.

The membrane has pores to allow the passage of fluid, either air orliquid, while retaining particulates such as fine dust. The porousnature of the membrane maintains differential pressure rise of less than0.7 MPa, e.g., less than 0.5 MPa or less than 0.1 MPa, described interms of the ISO-test method.

In one embodiment, the porous membrane is an ePTFE membrane having atight microstructure having a bubble point that is 0.06 MPa or more,e.g., 0.1 MPa or more or 0.5 MPa or more. In terms of ranges, the bubblepoint is from 0.06 to 1 MPa, e.g., from 0.06 to 0.7 MPa or from 0.07 to0.5 MPa. The ePTFE membranes may also have an average pore diameter thatis 0.7 microns or less, e.g., 0.5 microns or less.

The ePTFE membrane may have a structure of a plurality of nodes and/or aplurality of fibrils that creates pores. In some embodiments, the ePTFEmembrane is substantially fibrillated and contains a few nodes. The useof PTFE provides good heat resistance and chemical inertness. PorousePTFE provides increased strength and stability that has a hightransverse strength to withstand the stress caused by pulse jetcleaning. These stresses on the membrane are of two types. One caused bythe sudden expansion during pulse jet cleaning. The other caused bysudden collapsing of the bag against its support substructure. Thestresses are incurred repeatedly over the lifetime of the filter bag,which may be several years. For some applications of the filter bag,there may be more than a million pulse cycles in each year ofoperations.

The membranes used herein have a bubble point that is 0.06 MPa or moreand a transverse strength that is 100 N/m or more to withstandstructural failures, such as cracks, tears, or ruptures, uponapplication of an air flow to release the collected particulates fromthe upstream surface. For ePTFE membranes, high transverse strength maybe achieved by biaxial stretching of an extruded PTFE tape.

Although the strength of the membranes in the transverse direction maybe high enough to prevent structural failures, increased strength in thelateral direction is not required. The lateral direction extends alongthe length of the filter bag and is angled or perpendicular to thetransverse direction. In one embodiment, the lateral strength is lessthan 100 N/m, e.g., less than 75 N/m or less than 50 N/m. Althoughlateral strength may be higher, it is not necessary to impart thebenefits on the filter bag.

The use of the porous ePTFE membrane provides higher air flow rates,i.e. high permeability, suitable for good air filtration. The membranesare porous and have an air permeability of at least 1 cfm/ft² @ 0.5 inchgauge of water, e.g., at least 5 cfm/ft² @ 0.5 inch gauge of water or atleast 10 cfm/ft² @ 0.5 inch gauge of water. Although low airpermeability was previously used with liquid streams, the presentinventors have found that a low air permeability membrane may be used ina filter bag with a higher bubble point.

A lightweight membrane may maintain the air flow and reduce the overallweight of the filter bag. Thus, in one embodiment there is provided amembrane having a mass per area ratio of 4.5 gsm or less, e.g., 4 gsm orless, 3 gsm or less or 1 gsm or less. Incidentally, the membrane mayhave a mass per area ratio range from 0.5 gsm to 4.5 gsm, e.g., from 0.5gsm to 4 gsm, or from 0.5 to 3 gsm. Membranes having a mass per arearatio of greater than 5 gsm are heavier and tend to have reducedpermeability that decreases the air flow.

Thinner membranes are generally lightweight and have good airpermeability. In one embodiment, the membrane has a thickness from 5 to50 microns, e.g., from 5 to 30 microns or from 10 to 30 microns. In someembodiments, membranes may be layered to achieve the desired thickness.

Although the membranes of the present invention have a high transversestrength, the membranes have a ball burst of 1.36 kg or less, e.g., 1.26kg or less, or 1.16 kg or less. A reduced ball burst allows the membranewith a bubble point of greater than 0.06 MPa to maintain airflow with alower rise in pressure differential. In addition, since the membrane isattached to the filter media, a reduced ball burst is desirable with thehigher transverse strength.

In one embodiment, there is provided a cleanable filter bag assembly forfiltering particulates from a gas stream comprising a supportsubstructure, and a filter bag at least partially surrounding thesupport substructure, in which the filter bag comprises a porousmembrane having an upstream surface exposed to the gas stream and afilter media attached to the porous membrane. The porous membrane has astrength in a transverse direction that is 100 N/m or more, and at leasttwo of the following properties: an air permeability of at least 1cfm/ft² @ 0.5 inch gauge of water, bubble point of 0.06 MPa or more, amass per area ratio of 4.5 gsm or less, a thickness from 5 to 50microns, and a ball burst of 1.36 kg or less. In a further embodiment,the porous membrane has bubble point of 0.06 MPa or more and a strengthin a transverse direction that is 100 N/m or more, and at least one ofthe following properties: an air permeability of at least 1 cfm/ft² @0.5 inch gauge of water, a mass per area ratio of 4.5 gsm or less, athickness from 5 to 50 microns, and a ball burst of 1.36 kg or less.

Filter Media

The membranes may be attached to the filter media using lamination,welding, sewing, tacking, clamping, or other suitable attachment means.In one embodiment, the membranes are adhered to the filter media using acontinuous layer of adhesive or a discontinuous adhesives of dots andgrids. A common adhesive is a fluorinated polymer adhesive, such as afluorinated ethylene propylene (FEP) copolymer, is usually coated ontothe filter media by transfer coating the top surface of the filter mediawith a PTFE dispersion or FEP aqueous dispersion. Other useful adhesivesinclude tetrafluoroethylene/perfluoropropylene copolymer, polyvinylidenedifluoride, and the like. In another embodiment, lamination of theporous ePTFE membrane to the coated side of the filter media is effectedby laying the membrane on the coated side of the filter media, andheating to above the adhesive melting point with light pressure. Inother embodiments, the filter media may comprise a PTFE or athermoplastic material and may be melted to membrane without requiring afurther adhesive.

In a baghouse, the filter media should be able to withstand hightemperatures without degradation. The filter media is a backer materialon to which the membrane may be adhered or laminated. Depending onchemical and/or moisture content of the gas stream, its temperature, andother conditions, filter bags may be constructed from fiberglass,polyester, cotton, nylon, or other materials. Felts and fibers made ofPTFE, polyester, polypropylene, polyphenylene sulfide, aramid, orpolyimide may also be used as the filter media. Commingling of fibersand filaments, such as fiberglass and PTFE, may also be employed asfilter media. For high temperature applications of greater than 400° C.,a woven PTFE, fiberglass or polyimide may be used as the filter media.

Commercially available PTFE fabrics are supported needlefelts of PTFEfiber. These felts usually weight from 650-900 g/m² and may bereinforced with a multifilament woven scrim (130-210 g/m²). The scrimelement can be made of any polytetrafluoroethylene, but preferably isexpanded, porous polytetrafluoroethylene. The felts are made up ofstaple fibers, (usually 6.7 denier/filament, or 7.4 dtex/filament) andare 5-20 cm in length.

In one embodiment, there may be a further layer added to the filtermedia, such as a wrap of a non-woven polypropylene material or anadsorbent component for capturing mercury on the downstream side of thefilter media. In another embodiment, the filter media may comprise oneor more catalysts for converting contaminants such as NOx, NH3, CO,dioxin, furan, or ozone. The catalyst may comprise an active materialsuch as TiO₂, V₂O₃, WO₃, MnO₂, Pt or Al₂O₃. With a catalyst,contaminants such as dioxins, furans, NOx, CO, and the like, can beeffectively removed from a fluid stream.

FIG. 3 is a filter assembly 100 comprising a filter bag 102 attached toa shaker mechanism (not shown). For illustration purposes, the layers ofthe filter bag 102 are cut-away. Filter bag 102 has a cap 116 at one endand an opening 108 at the opposite end to allow cleaned air to passthrough and out. The filter assembly 100 include a wire 118 for mountingthe filter assembly 100 to a shaker mechanism (not shown). The filterbag 102 comprises a porous membrane 120, as described herein, that isattached to filter media 122 using an adhesive layer 124. The adhesivelayer 124 may be a continuous layer or a grid, lines or pattern ofadhesive forming a discontinuous layer. The porous membrane 120 has abubble point of 0.06 MPa or more and a strength in the transversedirection (t) that is 100 N/m or more. As shown in FIG. 3, the porousmembrane 120 is shown on the internal surface of the filter media 122.

Support Substructures

Support substructures that may be used in the filter elements of thepresent invention can vary widely depending on a number of conditions,including the configuration of the filter assembly, the type of materialto be filtered, the filtration system into which the filter assemblywill be incorporated, cleaning mechanism, and the like. For example,suitable support structures for use in the present application includecages, rings, or braces that may be fabricated from materials such asmetals, plastics, and natural fibers, including woven or nonwoven forms,such as spunbonded polyester or nonwoven aramid felt materials. In oneembodiment, the support substructure may be metal or plastic meshes.Wire supports cages may also be used as support substructures. In otherembodiments, the support substructure may be a rigid self-supportinginsert.

In one embodiment, the support substructure comprises a cage that may beconstructed as one, unitary piece or assembled from multiple pieces. Thecage may have a cover to provide a barrier between the cage and thefilter media. The cover may reduce contact between the filter media andthe support substructure.

FIG. 4 is a filter assembly 100 comprising a filter bag 102 fittedaround a support substructure 104, which is shown as a metal cage. Forillustration purposes, the layers of the filter bag 102 are pulled down,but it should be understood that these layers cover the supportsubstructure 104. As shown in FIG. 3, the support substructure 104 is awire cage that is used for pulse-jet cleaning. Filter bag 102 has a cuffportion 106 at one end and an opening 108 at the opposite end to allowcleaned air to pass through and out. The filter assembly 100 include aretaining portion 110 for mounting a bag to a tube sheet. In otherembodiments, the filter assembly may have a hook or other fastener forsecuring the filter assembly 100 within the baghouse. The filter bag 102comprises a porous membrane 120, as described herein, that is attachedto filter media 122 using an adhesive layer 124. The porous membrane 120shown in FIG. 4 has a bubble point of 0.06 MPa or more and a strength inthe transverse direction (t) that is 100 N/m or more.

Sacrificial Material

Although the porous media is shown as attached directly to the filtermedia in FIG. 4, in other embodiments, there may be a sacrificialmaterial disposed between the filter media and the porous membrane. Thissacrificial material prevents the abrasive topology of the filter mediafrom causing structural failures in the porous membrane. The sacrificialmaterial protects the porous media without causing a loss in particulatecollection. In one embodiment, there is provided a cleanable filter bagassembly for filtering particulates from a gas stream comprising asupport substructure, and a filter bag at least partially surroundingthe support substructure comprising a laminate of a membrane and asecond layer comprising a sacrificial material. The laminate may includean upstream surface of the porous membrane exposed to the gas stream,while the second layer is attached to the filter media.

The second layer may comprise a sacrificial material to create a smoothsurface for adhering the membrane to the filter media. Due to thesurface roughness of the filter media, the filter media may puncture themembrane when attached directly together. In one embodiment, the secondlayer reduces this occurrence by providing a barrier between themembrane and filter media. The second layer may be punctured by thefilter media without causing damage to the porous membrane. Thepuncturing may occur during assembly or during cleaning. Thus, theporous membrane retains its particulate retention and continues tooperate without suffering structural issues that detract from thedurability.

In this embodiment, the porous membrane attached to a second layer has astrength in a transverse direction that is 50 N/m or more, e.g., 60 N/mor more or 100 N/m or more. In this embodiment, the porous membrane mayhave a relatively lower transverse strength as compared to embodimentsthat use a porous membrane attached to the filter media due to thepresence of the second layer between the porous membrane and filtermedia. In addition, when the porous membrane is used as a laminate, thebubble point of the porous membrane is 0.06 MPa or more, e.g., 0.2 MPaor more or 0.5 MPa or more.

The second layer may be a woven textile, a non-woven textile, or amembrane. In one embodiment, a membrane such as a fluoropolymer membraneor a polyester membrane may be used as the second layer. A suitablemembrane for the second layer may be porous having a permeability of 1Frazier or more, e.g., 5 Frazier or more. When laminated to the porousmembrane, the laminate may have a permeability of 2 Frazier or more. Thesize and weight of the second membrane layer may vary depending on thesacrificial material. In one embodiment, the second membrane layer has amass to area ratio from 0.25 to 50 gsm.

In other embodiments, the sacrificial layer can be formed from a varietyof conventional fibers including, but not limited to, cellulosic fiberssuch as cotton, hemp or other natural fibers, inorganic fibers includingglass fibers, carbon fibers or organic fibers such as polyesters,polyimides, polyamides, polyolefins, or other conventional fibers orpolymeric materials and mixtures thereof.

The sacrificial materials in the laminate can be woven or non-woven. Inwoven bags, the fibers are typically formed into an interlocking mesh offiber in a typical woven format. Non-woven fabrics are typically made byloosely forming the fibers, in no particular orientation, and thenbinding the fibers into a filter fabric. One mode of constructing thesecond layer is using a felt media as a substrate. Felts are acompressed, porous, non-woven fabric made by laying discrete natural orsynthetic fibers and compressing the fibers into a felt layer usingcommonly available felt bonding technology that would be known to oneskilled in the art.

The filter assembly 100 shown in FIG. 5 comprises a laminate 130 of aporous membrane 120 and a second layer 132 that comprises a sacrificialmaterial. A filter bag 102 is fitted around a support substructure 104,which is shown as a metal cage. Similar to FIG. 4, for illustrationpurposes, the layers of the filter bag 102 and laminate 130 are pulleddown, but it should be understood that these layers cover the supportsubstructure 104. Similar to FIG. 4, a cuff portion 106 is shown at oneend and an opening 108 at the opposite end to allow cleaned air to passthrough and out the filter bag 102. The filter assembly 100 has aretaining portion 110 for mounting a baghouse to a tube sheet. In otherembodiments, the filter assembly 100 may have a hook or other fastenerfor securing the filter assembly 100 within the bag house. The filterbag 102 comprises a laminate 130 that is attached to a filter media 122using an adhesive layer 124. The laminate has an upstream surface thatis the porous membrane 120 and a downstream surface that is the secondmaterial 132, as described above. In FIG. 5, the second material 132 isa membrane. When formed as a laminate, the porous membrane 120 has abubble point of 0.06 or more. In one embodiment, the porous membrane 120may also have a strength in the transverse direction (t) that is 50 N/mor more, e.g., 100 N/m or more or 175 N/m or more.

Seam Sealing Tape

The filter bag may comprise at least two adjacent edge portions joinedby a seam, and a seam tape comprising a layer of an expandedfluoropolymer and a material for adhering to and sealing the seam. Theseam tape may be disposed over the seam on the porous membrane upstreamsurface and/or on the downstream surface of the filter media.

In one embodiment, the seam sealing tape may be comprise an expandedfluoropolymer which has a crosswise direction matrix modulus of greaterthan about 1,950 psi at room temperature and an enthalpy ratio of lessthan 0.6, as described in U.S. Pat. No. 8,790,432, the entire contentsand disclosures of which is hereby incorporated by reference.

The seam joining the sheet of filter media can be formed by conventionalmethods, such as sewing or heat welding. The material, size, and natureof the thread used to join filter media portions to form the seam of thefilter depends upon the filter media, the other materials which may beused to form the filter, and the ultimate use of the filter. The mannerin which the seam on the filter bag is sewn may be varied. For example,the seam may be a felled seam, the seam may be straight-stitched with abinder strip inserted between the edges of the filter media prior tosewing the seam, or the seam may be over-stitched.

The filter bag provided by way of the present invention has a variety ofsuitable end uses. In particular, the filter bag may be used to filterpaints and coatings, especially water-based paints and primers,chemicals, petrochemical products, water, aqueous solutions andsuspensions, and the like. The filter bag of the invention may be usedin the production of minerals, chemicals, metals and energy. Mostparticularly, the filter bag having a membrane with a high transversestrength can be used to filter and collect particulate dust emissions inbaghouses used in cement plants. The utility of the filter bag is in noway limited to these uses and includes most uses for conventional filterbags.

The present invention will be better understood in view of the followingnon-limiting examples and test results.

Test Methods

It should be understood that although certain methods and equipment aredescribed below, any method or equipment determined suitable by one ofordinary skill in the art may be utilized.

Transverse Strength

The strength in the transverse direction is determined according to thetest method of ASTM method D412-06a (2006), using die F. A universaltesting machine capable of constant rate of extension (CRE) control isused to test the membrane that is shaped as a dog bone or dumbbell. Todetermine transverse strength, the dumbbell-shaped sample is placed inthe grips. The rate of grip separation should be 500 to 50 mm/min. Themachine starts to apply a separation force and the force at rupture isrecorded.

Air Permeability—Frazier Number Determination

Air permeability of materials are determined according to test methodsentitled ASTM D 737-75, “Standard Test Method for AIR PERMEABILITY OFTEXTILE FABRICS.” Specifically, air permeability was measured byclamping a test sample in a gasketed flanged fixture which provided acircular area of approximately 6 square inches (3871 square mm) (2.75inches (70 mm) diameter) for air flow measurement. The upstream side ofthe sample fixture was connected to a flow meter in line with a sourceof dry compressed air. The downstream side of the sample fixture wasopened to the atmosphere.

Testing was accomplished by applying a pressure of 0.5 inches watergauge to the upstream side of the sample and recording the flow rate ofthe air passing through the in-line flowmeter (a ball-float rotameter).

Results are reported in terms of Frazier Number which is air flow incubic feet/minute/square foot of a sample at 0.5 inches water gauge.

Bubble Point Test

Liquids with surface free energies less than that of stretched porousPTFE can be forced out of the structure with the application of adifferential pressure. This clearing will occur from the largestpassageways first. A passageway is then created through which bulk airflow can take place. The air flow appears as a steady stream of smallbubbles through the liquid layer on top of the sample. The pressure atwhich the first bulk air flow takes place is called the bubble point andis dependent on the surface tension of the test fluid and the size ofthe largest opening. The bubble point can be used as a relative measureof the structure of a membrane and is often correlated with some othertype of performance criteria, such as filtration efficiency.

Bubble point was determined via a Capillary Flow Porometer, CFP-1500,made by Porous Materials Inc. Silwick oil was used as the wetting fluidto fill the pores of the tested membrane. The surface tension of thesilwick oil is 20.1 dyne/cm. The test area of the membrane is 2.84 cm².A metal support is placed beneath the sample test membrane to preventthe membrane from being deformed.

The Bubble Point is the pressure of air required to displace the wettingfluid from the largest pores of the test specimen and create the firstcontinuous stream of bubbles detectable by their rise through a layer ofwetting fluid covering the porous media. This measurement provides anestimation of maximum pore size.

Mass Per Area

The mass per area of samples was measured by the following method. Foursamples of the membrane using a D412F die (dogbone shaped) with an area2.03 in2 are punched using a clicker press. The mass of each of thesefour punched samples is measured and recorded in units of grams.

The mass of each sample is converted to mass/area in units of g/m² inthe following way. Mass/area (g/m2)=[mass of membrane sample punchedfrom Die D412F (g)×39.372]÷ 2.03 which simplifies to: mass/area(g/m2)=mass of membrane sample punched from Die D412F (g)×763.5. Theaverage value is reported as the mass/area of the membrane in units ofg/m².

Pressure Drop

The change in pressure drop is measured using method ISO 11057 (Firstedition; May 15, 2011). Where the residual pressure drop is taken 4seconds after a pulse. In the aging test method, the residual pressuredrop after cycle 1 and cycle 2500 are taken. The dP Rise is defined asthe difference between the residual pressure drops at cycle 2500 andcycle 1: dP Rise=Res dP_(Cycle 2500)−Res dP_(Cycle1). The lower rise inthe pressure drop indicates a more cleanable filter assembly.

Particle Capture

The filtration efficiency in capturing particles is determined using anAutomated Filter Tester Mod& 3160 (TSI). The flow rate for measuring is32 L/min. The 3160 challenges the filter with up to 20 differentmonodisperse particle sizes in the range from 15 to 800 nm. Thepenetration value for each particle size is calculated. At the end of atest, the Model 3160 generates a curve of penetration vs. particle sizeand produces a summary of test results, including the most penetratingparticle size (MPPS).

Ball Burst Strength

This test measures the relative strength of a sample of media bydetermining the maximum load at break. The media is challenged with a 1inch (2.54 cm) diameter ball while being clamped between two plates. TheChantillon, Force Gauge/Ball Burst Test was used.

The media is placed taut in the measuring device and pressure affixed byraising the web into contact with the ball of the burst probe. Pressureat break is recorded.

EXAMPLES Example 1

An expanded polytetrafluoroethylene (ePTFE) membrane having amass-to-area ratio of 2.2 gsm, permeability of 2.6 cfm/ft²/min and abubble point of 0.09 MPa was adhered to a PTFE felt without using anadhesive. Table 1 reports the durability and cleanability using apulse-jet cleaning technique. As indicated in Table 1, example 1exhibits acceptable durability with some damage and an excellentcleanability.

Example 2

A filter was prepared in a similar manner as Example 1. Table 1indicates the characteristics of the ePTFE membrane. As indicated inTable 1, example 1 exhibits acceptable durability with some damage andan excellent cleanability.

Comparative Example A

A filter was prepared in a similar manner as Example 1, but an ePTFEmembrane having a lower transverse strength as shown in Table 1. Due tothe lower transverse strength, comparative example A suffers from damageand has a limited durability as reported in Table 1.

Comparative Example B

A filter was prepared in a similar manner as Example 1, but an ePTFEmembrane having more open microstructure indicated by the lower bubblepoint of 0.013 MPa as shown in Table 1. Due to the open structure,comparative example B has poor cleanability as reported in Table 1 andthus an increased pressure drop.

TABLE 1 Trans- Mass- Perme- verse to-area ability Bubble Strength ratio(cfm/ft²/ Point # (N/M) (GSM) min) (MPa) Durability Cleanability 1 1242.2 2.6 0.09 Acceptable Excellent 2 200 4.1 1.9 0.09 AcceptableExcellent Comp. A 94 1.62 2.5 0.15 Damaged Excellent Comp. B 168 3.517.5 0.013 Excellent Not acceptable

The rise in differential pressure is compared between example 1 or 2 andcomparative example A or B and shown in FIG. 6. The single layermembrane having a tight microstructure and higher transverse strengthshows a significant improvement by having a rise in differentialpressure of 100 Pa (example 1) and 92 Pa (example 2). In contrastcomparative example A that has damage had a differential pressure riseof 93 Pa, while comparative example B had good durability but adifferential pressure rise of 148 Pa.

Examples 3 and 4

Laminates comprising a porous membrane as the top layer (facingupstream) and a sacrificial layer (bottom layer) adjacent to the filtermedia. The mass-to-area ratio and permeability of each layer arereported in Table 2. The filter media for these examples was 22 oz wovenfiberglass and the laminate was adhered using a PTFE dispersion coating.The transverse strength of the porous membrane was greater than 50 N/m.Table 2 reports the durability and cleanability using a pulse-jetcleaning technique. Both examples 3 and 4 exhibits excellent durabilityand cleanability.

TABLE 2 Top Layer Bottom Layer Mass- Perme- Mass- Perme- Bubble to-areaability to-area ability Point ratio (cfm/ft²/ ratio (cfm/ft²/ # (MPa)(GSM) min) (GSM) min) Durability Cleanability 3 0.09 3.27 2.3 4.8 40Excellent Excellent 4 0.21 1 3.5 4.8 40 Excellent Excellent

FIG. 7 is a graph showing the rise in differential pressure for alaminate in Example 3 of 120 Pa and for Example 4 is 42 Pa. This is asignificant improvement over a conventional filter.

The invention has now been described in detail for the purposes ofclarity and understanding. However, those skilled in the art willappreciate that certain changes and modifications may be practicedwithin the scope of the appended claims.

In the preceding description, for the purposes of explanation, numerousdetails have been set forth in order to provide an understanding ofvarious embodiments of the present invention. It will be apparent to oneskilled in the art, however, that certain embodiments may be practicedwithout some of these details, or with additional details.

Having disclosed several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theembodiments. Additionally, a number of well-known processes and elementshave not been described in order to avoid unnecessarily obscuring thepresent invention. Accordingly, the above description should not betaken as limiting the scope of the present invention or claims.

Where a range of values is provided, it is understood that eachintervening value, to the smallest fraction of the unit of the lowerlimit, unless the context clearly dictates otherwise, between the upperand lower limits of that range is also specifically disclosed. Anynarrower range between any stated values or unstated intervening valuesin a stated range and any other stated or intervening value in thatstated range is encompassed. The upper and lower limits of those smallerranges may independently be included or excluded in the range, and eachrange where either, neither, or both limits are included in the smallerranges is also encompassed within the present invention, subject to anyspecifically excluded limit in the stated range. Where the stated rangeincludes one or both of the limits, ranges excluding either or both ofthose included limits are also included.

As used herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural references unless the context clearly dictatesotherwise. Also, the words “comprise,” “comprising,” “contains,”“containing,” “include,” “including,” and “includes,” when used in thisspecification and in the following claims, are intended to specify thepresence of stated features, integers, components, or steps, but they donot preclude the presence or addition of one or more other features,integers, components, steps, acts, or groups.

In the following, further examples are described to facilitateunderstanding of the disclosure:

E1. A filter assembly for filtering particulates from a gas streamcomprising: a filter bag comprising a filter media and a porous membranehaving an upstream surface exposed to the gas stream and a downstreamsurface adjacent to the filter media, wherein the porous membrane has abubble point of 0.06 MPa or more and wherein the porous membrane has astrength in a transverse direction that is 100 N/m or more, wherein thefilter assembly is cleanable.

E2. The filter assembly of example E1, wherein the filter mediacomprises a woven felt, non-woven felt or a fiberglass material.

E3. The filter assembly of any one of examples E1 or E2, wherein theporous membrane has a structure to collect the particulates on theupstream surface.

E4. The filter assembly of example E3, wherein the porous membrane iscapable of withstanding structural failures caused by stresses of acleaning process to release the collected particulates from the upstreamsurface of the porous membrane.

E5. The filter assembly of any one of examples E1-E4, wherein the porousmembrane comprises a fluoropolymer membrane or a polyester membrane.

E6. The filter assembly of any one of examples E1-E5, wherein the porousmembrane has a strength in the transverse direction that is 175 N/m ormore.

E7. The filter assembly of any one of examples E1-E6, wherein the porousmembrane has a mass per area ratio of 4.5 gsm or less.

E8. The filter assembly of any one of examples E1-E7, wherein the porousmembrane has a Ball Burst strength of 1.36 kg or less.

E9. The filter assembly of any one of examples E1-E8, wherein the porousmembrane has a permeability of 1 cfm/ft² @ 0.5 inch gauge of water.

E10. The filter assembly of any one of examples E1-E9, wherein theporous membrane has a thickness from 5 to 50 microns.

E11. The filter assembly of any one of examples E1-E10, wherein theporous membrane has a bubble point of 0.09 MPa or more.

E12. The filter assembly of any one of examples E1-E11, wherein theporous membrane collects more than 97% of the particulates having adiameter of greater than 0.07 microns from the gas stream.

E13. A baghouse filter system comprising: a housing having an inlet andan outlet; a tube sheet positioned within the housing between the inletand outlet; and one or more of the filter bag assemblies of any one ofexamples E1-E12 mounted to the tube sheet.

E14. A filter assembly for filtering particulates from a gas streamcomprising: a filter bag comprising a filter media, a porous membranehaving an upstream surface exposed to the gas stream, and second layer,wherein the second layer is disposed between the filter media and porousmembrane, wherein the porous membrane has a bubble point of 0.06 MPa ormore, wherein the filter assembly is cleanable.

E15. The filter assembly of examples E14, wherein the porous membranehas a bubble point of 0.09 MPa or more.

E16. The filter assembly of any one of examples E14 or E15, wherein thesecond layer has a permeability of 10 cfm/ft² @ 0.5 inch gauge of water.

E17. The filter assembly of any one of examples E14-E16, wherein thesecond layer and the porous membrane are laminated together to form alaminate.

E18. The filter assembly of any one of examples E14-E17, wherein thesecond layer comprises a woven textile, a non-woven textile, or amembrane.

E19. The filter assembly of any one of examples E14-E18, wherein thesecond layer is laminated to the filter media.

E20. The filter assembly of any one of examples E14-E19, wherein thefilter media comprises a woven felt, non-woven felt or a fiberglassmaterial.

E21. The filter assembly of any one of examples E14-E20, wherein theporous membrane has a structure to collect the particulates on theupstream surface.

E22. The filter assembly of example E21, wherein the porous membrane iscapable of withstanding structural failures caused by stresses of acleaning process to release the collected particulates from the upstreamsurface of the porous membrane.

E23. The filter assembly of any one of examples E14-E22, wherein theporous membrane comprises a fluoropolymer membrane or a polyestermembrane.

E24. The filter assembly of any one of examples E14-E23, wherein theporous membrane has a strength in the transverse direction that is 50N/m or more.

E25. The filter assembly of any one of examples E14-E24, wherein theporous membrane has a strength in the transverse direction that is 100N/m or more.

E26. The filter assembly of any one of examples E14-E25, wherein theporous membrane has a strength in the transverse direction that is 175N/m or more.

E27. The filter assembly of any one of examples E14-E26, wherein theporous membrane has a mass per area ratio of 4.5 gsm or less.

E28. The filter assembly of any one of examples E14-E27, wherein theporous membrane has a Ball Burst strength of 1.36 kg or less.

E29. The filter assembly of any one of examples E14-E28, wherein theporous membrane has a permeability of 1 cfm/ft² @ 0.5 inch gauge ofwater.

E30. The filter assembly of any one of examples E14-E29, wherein theporous membrane has a thickness from 5 to 50 microns.

E31. The filter assembly of any one of examples E14-E30, wherein theporous membrane collects more than 97% of the particulates having adiameter of greater than 0.07 microns from the gas stream.

E32. The filter assembly of any one of examples E14-E31, is cleaned by apulse jet, reverse air, or shaker cleaning technique.

E33. A baghouse filter system comprising: a housing having an inlet andan outlet; a tube sheet positioned within the housing between the inletand outlet; and one or more of the filter bag assemblies of any one ofexamples E14-E32 mounted to the tube sheet.

1-20. (canceled)
 21. A filter assembly comprising: a filter bagcomprising: a filter media; a porous membrane, wherein the porousmembrane includes an upstream surface configured to be exposed to a gasstream, wherein the porous membrane has a bubble point of 0.06 MPa ormore, and wherein, when the porous membrane is exposed to dust andtested in accordance with ISO 11057:2011, the porous membrane exhibits arise in differential pressure of less than 100 Pa between an initialtest cycle and a final test cycle; and a second layer, wherein thesecond layer is disposed between the filter media and the porousmembrane, wherein the second layer comprises a membrane having apermeability of 1 Frazier or more and a mass to area ratio from 0.25 to50 gsm, wherein the filter assembly is configured to filter particulatesfrom the gas stream.
 22. The filter assembly of claim 21, wherein theporous membrane has a bubble point of 0.2 MPa, a transverse strength of91 N/m and a thickness of 8 microns.
 23. The filter assembly of claim21, wherein the filter media comprises a woven felt, a non-woven felt ora fiberglass material.
 24. The filter assembly of claim 21, wherein theporous membrane comprises a fluoropolymer membrane or a polyestermembrane.
 25. The filter assembly of claim 21, wherein the porousmembrane has a strength in the transverse direction that is 50 N/m ormore.
 26. The filter assembly of claim 21, wherein the porous membranehas a mass per area ratio of 4.5 gsm or less.
 27. The filter assembly ofclaim 21, wherein the porous membrane has a Ball Burst strength of 1.36kg or less.
 28. The filter assembly of claim 21, wherein the porousmembrane has a permeability of 1 cfm/ft2 @ 0.5 inch gauge of water. 29.The filter assembly of claim 22, wherein the porous membrane has apermeability of 4.2 cfm/ft2 @ 0.5 inch gauge of water.
 30. The filterassembly of claim 21, wherein the porous membrane has a thickness offrom 5 to 50 microns.
 31. The filter assembly of claim 21, wherein thesecond layer has a permeability of 10 cfm/ft2 @ 0.5 inch gauge of water.32. The filter assembly of claim 21, wherein the second layer and theporous membrane are laminated together to form a laminate.
 33. Thefilter assembly of claim 21, wherein the second layer has a permeabilityof 25 cfm/ft2 @ 0.5 inch gauge of water.
 34. The filter assembly ofclaim 21, wherein the second layer is laminated to the filter media. 35.The filter assembly of claim 21, wherein the porous membrane isconfigured to collect more than 97% of the particulates having adiameter of greater than 0.07 microns from the gas stream.
 36. Abaghouse filter system comprising: a housing having an inlet and anoutlet; a tube sheet positioned within the housing between the inlet andoutlet; and one or more of the filter bag assemblies of claim 1 mountedto the tube sheet.
 37. The filter assembly of claim 21, wherein thefilter media comprises a catalytic filter media.